Modulating the polyoxometalate-based inorganic–organic hybrids from simple chains to complicated frameworks via changing POM clusters

Abstract

Investigation into a hydrothermal reaction system with a copper salt, 1-(imidazo-1-ly)-4-(1,2,4-triazol-1-ylmethyl)benzene) (itb) ligand and three types of polyoxometalates (POMs) led to the preparation of three new coordination polymers, [Cu₂(itb)₄(H₄P₂W₁₈O₆₂)]·H₂O (1), [Cu(itb)₄(HPW₁₂O₄₀)]·4H₂O (2) and [Cu₂(itb)₂(Mo₈O₂₆)]·6H₂O (3). The three coordination polymers were characterized by elemental analyses, single-crystal X-ray diffraction, powder X-ray diffraction, XPS and IR spectra. Structural analyses show that with modulating the type of POM anions from [Mo₈O₂₆]⁴⁻ and [PW₁₂O₄₀]³⁻ to [P₂W₁₈O₆₂]⁶⁻, the three coordination polymers possess distinct structural motifs: simple one-dimensional (1D) chain (1), 1D + 1D → 2D interdigitated architectures (2) and complicated 3D frameworks consisting of both meso-helixes and left/right-helixes (3). The distinct structural features of the three coordination polymers suggest that the POM anions should play a significant role in the process of assembly. The electrocatalytic and luminescent properties for coordination polymers 1–3 have been investigated.

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Introduction

Polyoxometalates (POMs), as a large family of metal-oxygen clusters, constitute a fascinating class of inorganic systems that is incomparable in structural diversity as well as in a wide range of potential applications in catalysis,¹ electrochemistry,² photochromism,³ magnetism,⁴ and medicine.⁵ By virtue of their special properties, the POM anions as a well-defined library of inorganic building blocks can link variable transition-metal complexes (TMCs) to construct various hybrid coordination polymers, so called POM-based inorganic–organic hybrid materials. The combination of POMs and TMCs has not only led to more complicated and fascinating structural topologies, but also exerted synergetic effects and merged the merits of both aspects, which reveal potential applications in many fields, especially in catalysis and electrochemistry.⁶ In this context, the POM-based inorganic–organic hybrid materials have gained a great deal of consideration, and many efforts have been devoted to their preparation over the past few decades.⁷

Hydrothermal technique is a powerful method for the preparation of the POM-based hybrid materials.⁸ Nevertheless, the rational design and synthesis of exact desired POM-based hybrid materials is still a challenge, considering that the final structures of hybrid coordination polymers are frequently modulated by various factors especially in the black-box hydrothermal environment. The POMs exhibit a wide variety of robust structural motifs of different topologies and charge.⁹ Thus, the POM anion is a key factor which may influence structures of POM-based hybrids.¹⁰ To address relationships between the final structures of hybrids and the POM anions is desirable, which may provide guidance or new strategies for ongoing synthetic work. On the other hand, the judicious choice of organic ligands is also an important factor in designing POM-based hybrid materials under hydrothermal conditions. Organic ligands play a crucial role in the assembly of their structural features and chemical properties, which determine potential functionalities of the eventual hybrids. Among numerous ligands, the asymmetric flexible ligand, 1-(imidazo-1-ly)-4-(1,2,4-triazol-1-ylmethyl)benzene (itb) (Chart S1†) captures our attention, since they possess the following virtues: (i) the itb ligand can provide three potential coordination nodes and show various coordination modes (Scheme S1†), which may endow the final hybrid networks with variable structural topologies; (ii) the flexible nature of –CH₂– spacers allows the imidazole and/or triazole groups to bend and rotate freely as bridging ligands when coordinating to metal centers so as to conform to the coordination geometries of metal ions.¹¹

Based on the above considerations, in order to obtain new POM-based hybrid coordination polymers as well as further explore the influence of POM anions on the process of assembling, we synchronously introduce three kinds of POM clusters, namely [Mo₇O₂₄]⁶⁻, [PW₁₂]³⁻ and [P₂W₁₈]⁶⁻ polyanions into an identical copper-itb reaction system. Herein, three new coordination polymers were prepared by using this reaction system: [Cu₂(itb)₄(H₄P₂W₁₈O₆₂)]·H₂O (1), [Cu(itb)₄(HPW₁₂O₄₀)]·4H₂O (2) and [Cu₂(itb)₂(Mo₈O₂₆)]·6H₂O (3). X-ray crystal structure analyses reveal that, with modulating the kinds of POM anions from [Mo₈O₂₆]⁴⁻ and [PW₁₂O₄₀]³⁻ to [P₂W₁₈O₆₂]⁶⁻, the three coordination polymers possess distinct structural motives: simple chain (1), 1D + 1D → 2D interdigitated architecture (2) and complicated 3D framework coexisting of both meso-helixes and left/right-helixes (3). The influence of the POM anions on the structural assembly was discussed. The electrochemical and photoluminescent properties for 1–3 have also been investigated in detail.

Experimental section

Materials and general methods

α-K₆P₂W₁₈O₆₂·15H₂O were prepared according to the reported procedures and verified by IR spectrum.¹² Elemental analyses of C, H and N were performed on a Perkin-Elmer 2400 CHN Elemental Analyzer and that of P, Cu, Mo and W were analyzed on a PLASMASPEC(I) ICP atomic emission spectrometer. The FT-IR spectra were recorded from KBr pellets in the range 4000–400 cm⁻¹ with a Nicolet AVATAR FT-IR360 spectrometer. A CHI660 electrochemical workstation was used for control of the electrochemical measurements and data collection. A conventional three-electrode system was used, with a carbon paste electrode (CPE) as a working electrode, a commercial Ag/AgCl as reference electrode and a twisted platinum wire as counter electrode. The powder X-ray diffraction (PXRD) patterns were recorded on a Siemens D5005 diffractometer with Cu-Kα (λ = 1.5418 Å) radiation. X-ray photoelectron spectroscopy (XPS) analyses were performed on a VGE scalab 250 spectrometer with an Al-Kα achromatic X-ray source.

Synthesis of [Cu₂(itb)₄(H₄P₂W₁₈O₆₂)]·H₂O (1)

A mixture of α-K₆P₂W₁₈O₆₂·15H₂O (1 g, 0.2 mmol), Et₃N (triethylamine) (0.2 mL, 1.386 mol), CuCl₂·2H₂O (0.16 g, 1 mmol), itb (0.18 g, 0.75 mmol) and water (15 mL) was stirred for 1 h. The resulting solution was transferred to a Teflon lined autoclave and kept under autogenous pressure at 160 °C for 4 days. After slow cooling to room temperature, brown block crystals of 1 were filtered and washed with distilled water (30% yield based on W). Anal. calcd for C₄₈H₅₀Cu₂N₂₀O₆₃P₂W₁₈: H, 0.93; C, 10.65; N, 5.17; P, 1.14; Cu, 2.35; W, 61.13%; found: H, 0.97; C, 10.70; N, 5.11; P, 1.19; Cu, 2.44; W, 61.05%.

Synthesis of [Cu(itb)₄(HPW₁₂O₄₀)]·4H₂O (2)

The synthetic method was similar to that of coordination polymer 1, except that the α-K₆P₂W₁₈O₆₂·15H₂O was replaced by H₃PW₁₂O₄₀·12H₂O (0.6 g, 0.2 mmol). Green block crystals of 2 were filtered, washed with water, and dried at room temperature (33% yield based on W). Anal. calcd for C₄₈H₅₃CuN₂₀O₄₄PW₁₂: H, 1.36; C, 14.73; N, 7.16; P, 0.79; Cu, 1.62; W, 56.36%; found: H, 1.43; C, 14.77; N, 7.08; P, 0.83; Cu, 1.67; W, 56.28%.

Synthesis of [Cu₂(itb)₂(Mo₈O₂₆)]·6H₂O (3)

The synthetic method was similar to that of coordination polymer 1, except that the α-K₆P₂W₁₈O₆₂·15H₂O was replaced by (NH₄)₆Mo₇O₂₄·4H₂O (0.5 g, 0.4 mmol). Blue block crystals of 3 were filtered, washed with water, and dried at room temperature (42% yield based on Mo). Anal. calcd for C₂₄H₃₄Cu₂Mo₈N₁₀O₃₂: H, 1.83; C, 15.42; N, 7.49; Cu, 6.80; Mo, 41.06%; found: H, 1.91; C, 15.46; N, 7.54; Cu, 6.74; Mo, 41.13%.

Preparations of 1-, 2-, 3-CPEs

Coordination polymer 1-modified carbon paste electrode (1-CPE) was fabricated as follows:¹³ 48 mg of graphite powder and 8 mg of coordination polymer 1 were mixed and ground together by agate mortar and pestle to achieve a uniform mixture, and then 0.6 mL of paraffin oil was added with stirring. The homogenized mixture was packed into a glass tube with 1.2 mm inner diameter, and the tube surface was wiped with paper. Electrical contact was established with copper rod through the back of the electrode. In a similar manner, 2- and 3-CPE electrodes were made with corresponding coordination polymers 2 and 3, respectively.

X-ray crystallography

Data collection for 1–3 were performed on a Bruker SMART Apex CCD diffractometer with Mo-Kα monochromatic radiation (λ = 0.71073 Å) at 296 K. Absorption corrections were applied by using the multi-scan program SADABS.¹⁴ The structures were solved by direct methods, and non-hydrogen atoms were refined anisotropically by least-squares on F² using the SHELXTL program.¹⁵ The hydrogen atoms of organic ligands were generated geometrically for 1–3, while the hydrogen atoms of water molecules can not be found from the residual peaks and were directly included in the final molecular formula. A summary of the crystal data, data collection, and refinement parameters for 1–3 are listed in Table 1.†

Table 1 Crystal data and structure refinements for coordination polymers 1–3

Compound	1	2	3
a R1 = ∑||Fo| − |Fc||/∑|Fo|.b wR2 = ∑[w(Fo^2 − Fc^2)^2]/∑[w(Fo^2)^2]^1/2.
Empirical formula	C48H50Cu2N20O63P2W18	C48H53CuN20O44PW12	C24H34Cu2N10O32Mo8
Mr	5413.36	3914.78	1869.19
Crystal system	Orthorhombic	Monoclinic	Monoclinic
Space group	Pna21	C2/c	C2/c
a, Å	22.959(5)	22.315(5)	23.853(5)
b, Å	20.190(5)	14.904(5)	8.985(5)
c, Å	21.181(5)	24.753(5)	22.923(5)
β, deg	90.000	100.564(5)	103.685(5)
V, Å^3	9818(4)	8093(4)	4773(3)
Z	4	4	4
Dcalcd, g cm^−3	3.658	3.205	2.584
T/K	296(2)	296(2)	296(2)
μ/mm^−1	21.552	17.358	3.007
Refl. measured	48 560	20 344	12 459
Refl. unique	17 388	7206	4700
F(000), e	9584	7024	3544
Rint	0.0799	0.0786	0.0356
Final R indices [I ≥ 2σ(I)]	R1^a = 0.0475, wR2^b = 0.1090	R1^a = 0.0712, wR2^b = 0.1562	R1^a = 0.0638, wR2^b = 0.1663
Goodness-of-fit on F^2	1.028	1.122	1.050

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Results and discussion

In order to confirm the valence of metal atoms in 1–3, firstly, the bond valence sum calculations (BVS) have been done.¹⁶ The BVS calculations show that the oxidation states of Cu atoms in 1 are in the +I, while that in 2 and 3 are all in the +II oxidation state. These results are further confirmed by their coordination environments, crystal color and X-ray photoemission spectrum (Fig. S3†). Furthermore, the BVS calculations show that the oxidation states of W atoms in 1 and 2 and Mo atoms in 3 are in +VI oxidation state. Since 1 and 2 were isolated from acidic aqueous solution, four protons and one proton are attached to P₂W₁₈, PW₁₂ cluster to compensate charge balance, respectively, which is similar to case of [Cu(bimb)]₂(HPW₁₂O₄₀)·3H₂O.¹⁷ In addition, during the preparation process of compounds 1–3, Et₃N is a necessary starting material for obtaining the compounds, although it is not in the existence of final structures. The Et₃N in this reaction system may be acted as not only a mineralization agent but also a reducing agent.¹⁸ The oxidation state of Cu centers were changed from +II to +I in compound 1, which can be attributed to reduction effect from Et₃N reducing agent. Such phenomenon is often observed in the hydrothermal reaction systems containing both N-donor of organic molecules and Cu^II ions.¹⁹

Structural description of 1

Single crystal X-ray diffraction analysis reveals that 1 crystallizes in the orthorhombic space group Pna2₁ (no. 33), and consists of two basic subunits: a [Cu₂(itb)₄]²⁺ cation (Fig. 1a) and a α-[P₂W₁₈O₆₂]⁶⁻ polyanion (abbreviated as P₂W₁₈).

Fig. 1 (a) The [Cu₂(itb)₄]²⁺ cation in 1 and (b) the structural unit of 1. The hydrogen atoms and solvent water molecules are omitted for clarity.

In the [Cu₂(itb)₄]²⁺ cation, there are two crystallographically independent copper atoms (Cu1 and Cu2) (Fig. 1a). The Cu1 atom is four-coordination in a seesaw geometry defined by two oxygen atoms from one P₂W₁₈ anion and two nitrogen donors from two ligands (Fig. 1b and Table S1†). The bond lengths and angles around the Cu1 are in the range of 1.87–1.881 Å (Cu–N), 2.758–2.828 Å (Cu–O), 175.8° (N–Cu–N) and 76.97–105.54° (N–Cu–O), respectively. The Cu2 atom is five-coordination in a distorted pyramid-shaped geometry defined by two oxygen atoms from two P₂W₁₈ clusters and three nitrogen donors from three ligands (Fig. 1b and Table S1†). The bond lengths and angles around the Cu2 are in the range of 1.91–2.48 Å (Cu–N), 2.831–2.867 Å (Cu–O), 175.8° (N–Cu–N) and 76.97–105.54° (N–Cu–O), respectively. All of these bond lengths and bond angles are within the normal ranges observed in other Cu-containing complexes.²⁰ By these coordination modes, the adjacent Cu1 and Cu2 atoms and four itb ligands are linked together to form a [Cu₂(itb)₄]²⁺ complex (Fig. 1a). The P₂W₁₈ polyanion shows the well-known Wells-Dawson type structure consisting two [PW₉O₃₄]⁹⁻ units derived from the α-Keggin polyanion by removal of a set of three corner-shared WO₆ octahedra.²¹ Each of the P₂W₁₈ anion connects three copper atoms (Table S1†) from two neighboring [Cu₂(itb)₄]²⁺ complexes. Consequently, a one-dimensional (1D) chain is formed running along a axis (Fig. 2).

Fig. 2 Polyhedral and ball-and-stick representation of the 1D chain in compound 1.

Structural description of 2

Single crystal X-ray diffraction analysis reveals that 2 crystallizes in the monoclinic space group C2/c (no. 15), and consists of two basic subunits: a [Cu(itb)₄]²⁺ cation (Fig. 3a) and a protonated [HPW₁₂O₄₀]²⁻ anion (abbreviated as PW₁₂).

Fig. 3 (a) The [Cu(itb)₄]²⁺ cation in 2 and (b) the structural unit of 2. The hydrogen atoms and solvent water molecules are omitted for clarity.

In the [Cu₂(itb)₄]²⁺ cation, each of the copper atom is six-coordination in a near-octahedral geometry achieved by two oxygen atoms from two adjacent PW₁₂ anions and four nitrogen donors from four mono-coordinate ligands, displaying Jahn–Teller (JT) elongation axes with the two JT bonds being at least 0.2 Å longer than the other four bonds (Fig. 3a and Table S1†). The coordination shape around Cu1 likes a “propeller” along the b axis. The bond lengths and angles around the Cu ions are in the ranges of 2.015(16)–2.029(15) Å (Cu–N), 2.270(17)–2.28(2) Å (Cu–O), 86.4(6)–178.1(9)° (N–Cu–N) and 89.1(5)–90.9(5)° (N–Cu–O), respectively, which are within the normal ranges for six-coordinated copper system.²² The PW₁₂ anion exhibits a α-Keggin configuration (Fig. 3b and Table S1†). The central atom P is disorderly surrounded by a cube of eight oxygen atoms, with each oxygen site half-occupied.²³ The P–O lengths are in the range of 1.48(2)–1.51(2) Å and W–O lengths are in the range of 1.646(17)–1.729(17) Å for terminal oxygen atoms, 1.864(18)–1.915(16) Å for μ₂-bridging oxygen atoms and 2.40(3)–2.52(2) Å for μ₃-bridging oxygen atoms, respectively. All the bond lengths are in the normal ranges and in close agreement with those described in the literature.²⁴

A structural feature of 2 is its interdigitated architecture achieved by novel molecular zippers, which can be described in two steps: in the first step, each of the PW₁₂ clusters acts as a didentate linker connects two [Cu(itb)₄]²⁺ propeller-shaped complexes by sharing two opposite terminal oxygen atoms of the cluster while each of the [Cu(itb)₄]²⁺ complexes is connected by two PW₁₂ clusters. Consequently, a poly-pendant 1D infinite chain running along the b axis is formed by repeating these connections, in which the mono-coordinated itb molecules as pendants are approximately perpendicular to the chain. The distance between the adjacent itb pendants from two [Cu(itb)₄]²⁺ complexes is ca. 14.90 Å, which is a big gap (Fig. S1†). As is known, such large structural gaps are often occupied by solvent molecules or guest molecules to achieve the structural stabilization. Otherwise, the interdigitation phenomena may occur, that is, the gaps associated with one structure motif are occupied by one or more independent structure motifs to fill the spaces. Interestingly, in the structure of 2, these gaps of one chain are interdigitated by the protrudent mono-coordinated itb pendants from the adjacent chains to form zipper-like architecture, in which overhanging mono-coordinated itb arms act as teeth. In the second step, the gaps of each strip of zipper-likes are further interdigitated by itb pendants from adjacent two strips of identical molecular zippers. As a result, a 1D + 1D → 2D interdigitated architecture is achieved (Fig. 4). Complementary intermolecular hydrogen bondings existing between the two adjacent chains stabilize this structure (typical hydrogen bondings: C2–H2A⋯O6 = 3.304 Å, C12–H12A⋯O14 = 3.133 Å, C14–H14A⋯O8 = 3.173 Å).

Fig. 4 The crystal packing diagram showing the 1D + 1D → 2D interdigitated architecture.

Structure description of compound 3

Single crystal X-ray diffraction analysis reveals that 3 crystallizes in the monoclinic space group C2/c (no. 15), and consists of two crystallographically independent copper atoms (Cu1 and Cu2), one [β-Mo₈O₂₆]⁴⁻ polyanion (abbreviated as β-Mo₈), two itb ligands (itb1 and itb2) and six H₂O molecules. Both Cu1 and Cu2 are six-coordination in a near-octahedral geometry, but their coordination environments are entirely different. The Cu1 is coordinated by four oxygen atom from four water molecules and two nitrogen atoms from two adjacent ligands (Fig. 5 and Table S1†). The Cu2 is coordinated by two oxygen atoms from two coordinated water molecules, two nitrogen atoms from two different ligands and two oxygen atoms from two β-Mo₈ anions (Fig. 5). The bond lengths and angles around the Cu1 and Cu2 atoms are in the ranges of 1.984(11)–2.085(12) Å (Cu–N), 2.000(11)–2.400(9) Å (Cu–O), 178.5(6)–180.0(14)° (N–Cu–N) and 85.3(5)–94.7(5)° (N–1Cu–O). All of these bond lengths and bond angles are within the normal ranges observed in other Cu(II)-containing complexes.²²

Fig. 5 View of the structural unit of 3. All the hydrogen atoms are omitted for clarity.

A structural feature of 3 is its 3D framework coexisting of both meso-helixes and left/right-helixes, which can be described as follows: in the crystal structure, all of the two crystallographically unique itb ligands (itb1 and itb2) severed as bidentate ligands (Table S1†) link each other by bridging Cu^II ions in a –Cu1–itb1–Cu2–itb2– fashion to generate an extraordinary 1D [Cu₂(itb)₂]_n⁴ⁿ⁺ meso-helix chain along the a axis (Fig. 6c). Such a meso-helix structure was rarely reported, especially in POM system.²⁵ Further, the meso-helix chains are linked by two-connected β-Mo₈ clusters via Cu–O bonds to achieve a 2D layer (Fig. 6a, top). After further investigation of the 2D layer along the b axis, it is clear to see that there exist left/right-handed helixes with an identical pitch of ca. 8.985 Å consisted of β-Mo₈ clusters, copper atoms and itb ligands (Fig. 6a, bottom).

Fig. 6 View of the structural motifs in 3: (a) the 2D layer formed by the left-handed and right-handed helixes, (b) the 3D framework and (c) meso-helix.

Finally, the adjacent layers are fused together by sharing the meso-helix chain to form a 3D complicated framework (Fig. 6b and S2†). To the best of our knowledge, the compound coexisting of both meso-helixes and left/right-helixes has been observed only once,²⁶ therefore, the structure of compound 3 provides a new example for investigation and synthesis of such helical system. The topological analysis of the structure has been performed by considering Cu1 atom as a 2-connected node, Cu2 as a 4-connected node and β-Mo₈ anion as a 2-connected node, the 3D framework of 3 can be simplified as a 3D (2,2,4)-connected net with a (12⁴·16²) topology.

Influence of the polyoxoanions on the structures of compounds 1–3

The rational design and dimensionality control of the resulting POM-based inorganic–organic hybrids is still a challenge at present, considering that the final structures of hybrid compounds are frequently modulated by various factors especially in the black-box hydrothermal environment. Herein, in order to systemically understand the influence of POM anions in the process of assembling, we synchronously introduce three kinds of POM clusters, namely [Mo₇O₂₄]⁶⁻, [PW₁₂]³⁻ and [P₂W₁₈]⁶⁻ anions into an identical copper-itb reaction system, respectively, and obtained three distinct compounds (Scheme 1). We deduce that the structural distinction of 1–3 may mainly arise from the difference of the volume of POMs. With modulating the volume of POM units from the largest volume of the P₂W₁₈ cluster to the smallest Mo₈ cluster, the copper cations exhibit a stronger tendency to show the highest coordination number (six), thanks to the low steric hindrance of PW₁₂ and Mo₈ clusters; the copper cations with high coordination number can effectively link itb ligands to form complicated Cu–itb complexes (see Table S1†). Finally, the POMs synergistically connect various Cu–itb complexes to generate compounds with distinct structural topology (Scheme 1).

Scheme 1 Summary of the influences of the different POM clusters on the structures of 1–3.

Analyses of IR and PXRD measurements

As shown in Fig. S4,† the IR spectra exhibit the characteristic peaks at 1088, 9455, 907 and 793 cm⁻¹ in 1 as well as 1054, 959, 879 and 801 cm⁻¹ in 2, which are attributed to ν(P–O), ν(W=Ot), ν_as(W–Ob–W) and ν_as(W–Oc–W) of the P₂W₁₈ and PW₁₂ polyanions respectively. The IR spectrum exhibits the characteristic peaks at 954, 898, 855 and 708 cm⁻¹ in 3, which are attributed to ν(Mo=Ot) and ν_as(Mo–Oc–Mo) of Mo₈O₂₆ polyanions.²⁷ Additionally, the bands in the region of 1600 to 1200 cm⁻¹ could be ascribed to the character peaks of itb ligands in 1–3.

As shown in Fig. S5,† the powder X-ray diffraction (PXRD) patterns measured for the as-synthesized samples of 1–3 are all in good agreement with the PXRD patterns simulated from the respective single-crystal X-ray data, indicating the phase purities of the compounds 1–3.

Luminescent properties

The luminescence properties of 1–3, together with the free L ligands, are investigated in the solid state at room temperature. As shown in Fig. 7, the ligand itb exhibits the main emission peak at 404 nm when the excitation wavelength is 300 nm. Compounds 1–3 possess solid-state emission spectra, which show luminescent emission maximums at 406, 407 and 424 nm upon excitation wavelength 300 nm, respectively. As is known, Cu^II complexes do not contain d¹⁰ metal centers, but a series of fluorescent Cu^II complexes have been reported.²⁸ Density functional theory calculation indicates that the fluorescence of Cu^II coordination complexes may be mainly attributed to the coupling of both ligand-to-metal charge transfer (LMCT) and ligand-to-ligand charge-transfer (LLCT).²⁹ Obviously, the emission bands of compounds 1 and 2 are very similar to that of free ligand itb, and thus the origin of the emission of compounds can be mainly assigned to the LLCT (Fig. 7). However, compared to the free ligand itb, the emission spectrum of compound 3 are red-shifted, which may be due to the effect of the coordination between ligands and metal ions.³⁰ The compounds 1–3 are insoluble in common polar and nonpolar solvents, therefore they may be good candidates for potential solid-state luminescent materials.

Fig. 7 The emission spectra of compounds 1–3.

Cyclic voltammetry (CV)

The POM-modified carbon paste electrode (CPE) often show the abilities to undergo reversible mono- and/or multielectron redox processes, which endows them attractive electrochemical and electrocatalytic properties.³¹ Herein, the cyclic voltammograms of compounds 1-, 2- and 3-CPEs were investigated in 1 M H₂SO₄ aqueous solution. As show in Fig. 8, it can be seen that there are three pairs of reversible redox peaks with the mean peak potentials E_1/2 = (E_pa + E_pc)/2 (scan rate: 100 mV s⁻¹) are −595 (I–I′), −369 mV (II–II′) and −130 mV (III–III′) for 1-CPE, which can be ascribed to redox corresponds to three two-electron process of W centers in P₂W₁₈ polyanions.³² Different form 1-CPE, it can be seen that there are two pairs of reversible redox peaks with the mean peak potentials E_1/2 = (E_pa + E_pc)/2 (scan rate: 100 mV s⁻¹) at −567 mV (I–I′) and −372 mV (II–II′) for 2-CPE as well as 232 mV (I–I′) and 390 mV (II–II′) for 3-CPE, which can be ascribed to redox of W centers in PW₁₂ polyanions and Mo centers in Mo₈ polyanions, respectively.^33,2a In addition, there is a irreversible anodic peak with the potential of +31 mV for 1-CPE and +80 mV for 2-CPE, which is assigned to the oxidations of the copper centers.³⁴ However, the oxidation peak of copper centers is not observed for 3-CPE, perhaps due to their weak signals embedded in the redox peaks of Mo.³⁵

Fig. 8 Cyclic voltammograms of the CPEs (a for 1-CPE, b for 2-CPE and c for 3-CPE) in the 1 M H₂SO₄ aqueous solution (scan rate: 100 mV s⁻¹).

The electrocatalytic properties of 1-, 2- and 3-CPEs have also been investigated. As shown in Fig. 9, with addition of nitrite, the reduction peak currents increase gradually while the corresponding oxidation peak currents decrease. The nearly equal current steps for each addition of nitrite demonstrate stable and efficient electrocatalytic activities of the 1-, 2- and 3-CPEs (see Fig. S6a–c†).

Fig. 9 Reduction of nitrite (a for 1-CPE, b for 2-CPE and c for 3-CPE) in the buffer solution (scan rate: 100 mV s⁻¹). The concentrations (from inner to outer) are 0, 2, 4, 6, 8 mM for nitrite.

To compared the electrocatalytic activity of 1-, 2- and 3-CPEs for nitrite, the CAT (catalytic efficiency) of 1-, 2- and 3-CPEs can be calculated by using CAT formula.³⁶ As shown in Fig. 10, from the chart of the CAT, we found that the 2- and 3-CPEs have a good electrocatalytic effect toward the reduction of nitrite, which suggest that compounds 2 and 3 may have potential applications in detection of nitrite.

Fig. 10 Chart of the CAT vs. concentration of NO₂⁻.

Additionally, the stability experiments for 1-, 2- and 3-CPE have been investigated scanning for 40 cycles in 1 M H₂SO₄ solution. As shown in Fig. S7,† it can be seen that the electrode exhibits almost no loss in the current signal after 40 cycles, which suggests that catalyst of 1-, 2- and 3-CPEs have high stability.

Conclusions

In summary, three new POM-based inorganic–organic hybrids have been synthesized under an identical hydrothermal condition except introducing different POMs into the reaction system. The three compounds exhibit the structure motifs from a simple 1D chain, (1D + 1D → 2D) layer, to a complicated 3D framework coexisting of both meso-helixes and left/right-helixes respectively, which suggests that the POM anions should play a significant role in the process of assembly. To some extent, this work can enlighten us to discover new trends and relationships between the final structures and influence of POM building blocks, which may provide further guidance or new strategies for ongoing synthetic work.

Acknowledgements

This work was financially supported by the NSF of China (no. 21371041 and 21101045), the NSF of Heilongjiang Province (no. B201103) and innovative research team of green chemical technology in university of Heilongjiang Province, China.

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Footnote

† Electronic supplementary information (ESI) available: Additional figures, PXRD, XPS, IR and CV. See DOI: 10.1039/c4ra08058k

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